The present invention relates to methods for avoiding excessive loads placed on turbine casings by pipes attached to the casing. In particular, the invention also relates to methods for optimizing the piping load limits for a turbine casing to minimize deformation of the casing due to the loads.
Deformation of a steam turbine casing can allow steam to leak around the seals at the tips of turbine buckets. This leakage reduces the amount of steam available to work on the downstream turbine buckets. Deformation of the casing can distort the seals which are intended to prevent the leakages of steam and gases.
Seals within a steam turbine generally include teeth static casing structure that interlace with the bucket covers. The gap between the teeth on the casing and the teeth on the buckets is narrow to prevent steam leakage over the covers of the blades. A steam turbine also has inter-stage seals between the stages of turbine buckets. The inter-stage seals prevent steam leakage through the turbine diaphragm packing that around the rotor shaft, and between each bucket stages. Deformation of the casing can distort the seals, and allow steam to escape through the seals and into the casing.
Excessive forces due to pipe connections on a turbine casing can occur during the assembly or operation of a turbine. Pipe loads tend to be high during turbine startup when the pipes and turbine heat up. The heating of the pipes and turbine casing during startup results in differential thermal expansions in the pipes and casing. The differential expansions between the pipes and casing apply loads on the casing that distort the casing shell. During turbine startup, distortions in the casing typically exaggerate the turbine bucket and seal clearances.
Excessive piping loads can also distort the turbine casing during turbine transient operations. Piping loads during transients, especially when cooling occurs in the pipes, tend to distort the turbine casing to reduce the clearances between the seals and buckets. If these clearances become too small, the stationary seals may “rub-out” as they scrap against the rotating buckets. Seals that rub-out do not provide effective sealing as they allow excessive steam leakage during steady state turbine operating conditions. Accordingly, excessive piping loads may damage and distort the seals between the casing and the buckets such that turbine performance is degraded.
Piping load limits are imposed on the turbine design to avoid excessive piping loads that unduly distort the casing. In the past, these limits have been based on empirical and historical information regarding prior piping loads on similar turbine casing. It was common for piping load limits to be developed using simple calculations of the loads on the casing. Conventional turbine casing design techniques were not particularly helpful in identifying those key pipe connections that distort the turbine casing (seals) or in establishing load limits for pipe connections. Accordingly, there is a long felt need for methods to establish pipe load limits on turbine casings that avoid excessive casing distortion.